The present invention relates to an X-ray Computed Tomography system which has an X-ray source for generating an X-ray fan beam and a multiple element ring detector, X-ray detector elements of which are arranged in a ring-like shape, wherein an X-ray beam is emitted to an object to be examined located substantially at the center of the ring detector while the X-ray source is rotated substantially concentrically with the ring detector, and a tomographic image of the object is reconstructed using projection data from the ring detector portion located opposite the X-ray source with the object interposed therebetween.
A conventional X-ray CT system of this type is schematically shown in FIG. 1. The system includes X-ray source 1, ring detector 3, data acquisition system (DAS) 5, sorter 6, rotation controller 7, interpolater 8, data memory 9, convolver 10, back projector 11, image memory 12 and cathrode ray tube (CRT) display 13. Source 1, constituted by an X-ray tube, is driven and controlled by controller 7. Source 1 emits X-ray fan beam 4, toward object 2 while being rotated thereabout. Ring detector 3, concentric with the rotational center of source 1, is fixed outside the rotating path of source 1. Detector 3 is comprised of a large number of X-ray detector elements arranged to form a ring. Beam 4 transmitted through object 2, is incident on the detector elements of detector 3 which are opposite source 1 with object 2 interposed therebetween. X-rays incident on these detector elements are converted to an electrical signal having a magnitude corresponding to the light intensity. This electrical signal is then supplied to DAS 5.
A cylindrical graticule (not shown) is fixedly disposed concentric with detector 3. The graticule has transparent and opaque portions alternately arranged at angular intervals on a circle. The number of transparent or opaque portions thereof is twice the number of the detector elements in detector 3. The transparent or opaque portions of the graticule are detected by a photointerrupter (not shown) moved along the circle in synchronism with source 1. Light detected by the photointerrupter is converted to electrical pulses generated at a pitch half that of the array of the detector elements upon rotation of source 1. The detection system consisting of the graticule and photointerrupter is hereinafter referred to as a graticule sensor.
Electrical pulses from the graticule sensor are then supplied to controller 7. Controller 7 controls the rotation of source 1 in response to the pulses from the graticule sensor. These pulses are also supplied to DAS 5.
DAS 5 samples electrical signals from detector 3 in response to pulses from the graticule sensor, i.e., in response to the rotation of source 1, and converts them into digital signals. Digital signals from DAS 5 are then supplied to sorter 6. For each sampling, i.e., each projection, projection data is supplied to sorter 6 and serves as projection data (hereinafter referred to as source fan data) corresponding to X-ray paths (hereinafter referred to as a source fan) projected in a fan shape with source 1 at its center. Sorter 6 rearranges source fan data obtained by a plurality of successive projections to data (hereinafter referred to as a detector fan data) corresponding to the source fan (hereinafter referred to as a detector fan) expanded from each detector element. The detector fan data (hereinafter referred to as first detector fan data) from sorter 6 is then supplied to interpolater 8. Interpolator 8 interpolates the first detector fan data corresponding to the X-ray paths passing the positions (at equal angular intervals with respect to the center of rotation of source 1) of source 1 and the detector elements. Interpolator 8 then calculates second detector fan data corresponding to predetermined X-ray paths which are arranged at equal angular intervals as viewed from the respective detector elements. Memory 9 is connected to DAS 5, sorter 6 and interpolator 8 to help the respective processing thereof. The second detector fan data from interpolator 8 is supplied to convolver 10. Convolver 10 performs convolution as a kind of filtering of the second detector fan data so as to prevent an image from being blurred upon back projection thereof, thereby obtaining third detector fan data. The third detector fan data from convolver 10 is supplied to back projector 11. Projector 11 projects the third detector fan data back to memory 12, which stores a memory space corresponding to coordinates of a slice of object 2. Back projection by projector 11 is performed such that sequential third detector fan data is accumulated and written in correspondence with the X-ray path position. The third detector fan data accumulated in memory 12 therefore allows reconstruction of image data representing a slice of object 2. The reconstructed image is then read out from memory 12 and displayed on display 13.
The positional relationship between each detector element of detector 3 and the sampling point (the position of source 1 at the time of sampling) of DAS 5 in the conventional X-ray CT system will be described with reference to FIG. 2.
As shown in FIG. 2 for each of central points #1, #2, . . . #N of N detector elements of detector 3, some sampling points correspond to each other (located at the same angle with respect to the center of rotation of source 1) among 2N sampling points S.sub.l, S.sub.2, . . . S.sub.2N of DAS 5 (in this case, some sampling points do not correspond to each other).
When detector fan data is obtained, the sampling pitch depends on the pitch between points S.sub.l, S.sub.2, . . . S.sub.2N. Thus, the highest possible resolution is determined by this pitch.
U.S. Pat. No. 4,176,279 describes an X-ray CT system of a type (so-called third generation type) wherein an X-ray source for generating an X-ray fan beam and a detector for detecting the beam are arranged opposite each other in relation to the object to be examined, and the X-ray source and the detector are rotated together around the object. According to a known technique for improving image quality of the reconstructed image, the detector is offset by a 1/4 pitch of the detector elements, and the pitch of the projection data (X-ray path) to be sampled can be doubled.
However, in a CT system of the type shown in FIGS. 1 and 2, fixed detector 3 cannot be offset by a 1/4 pitch of the detector elements.